Transmission regulation



June 2, 1931.

J. C. GABRIEL ET AL TRANSMISSION REGULATION Filed March 11, 1929lNl/ENTUFS HUBBARQ Patented June 2, 1931 um'rso STATES mammen JOHN G.GABRIEL, O! m. YORK, AND IRAN! A. HUBBARD, 0] 13111183008, NEW YORK,

ASSIGNOBB 1'0 am TELEPHONE LABORATORIES, INCORPORATED, OI NEW YORK,

N. Y A. CORPORATION 01 NEW YORK armsinssron BIGI ILA'I'ION- Applicationfiled larch 11, 1828. Serial 110. 845,887.

This invention relates to radio receiving systems and more particularlyto means for maintaining at a constant level the output energy suppliedto'a load circuit.

Numerous gain control devices have been proposed which utilize differentcomponents of. the incoming wave to effect the desired control. One suchtype is that disclosed in Patent 1,511,015 to Affel, October "7, 1924,in which the unmodulated carrier component is utilized to effect thedesired control. Another type is disclosed in an application of E.Bruce, Serial No. 158,169, filed December 31, 1926, Patent 1,778,75Q,October 21 1930, in which the signal or audio frequency component or acomponent of changed frequency, derived from the received wave, may beused to effect the control function.

The present invention relates to asystem which embodies certain featuresof the abovementioned patent and application in combination with addedfeatures for improving and simplifying the operation of the gain controlmechanism.

An object of this invention is to efiect gain control which, whileextremely eflici'ent and having a small time lag, requires a minimumofmanual adjustment to regulate its operation.

Another object is to regulate the operation of the gain controlmechanism by a single adjustment, which fixes the relative potentialsapplied to a plurality of devices included in the receiver.

A feature of the invention relates to the provision, in a circuitincludin a space discharge tube thegrid of which 1s biased by apotential derived from a point of fixed potential in the filamentheating circuit, of

means whereby the potential difference between the grid and filament maybe changed without changing the potential of the fixed point.

The above objects are accomplished in one embodiment of the invention,comprising asuccessive detection receiving system, by util izingaportion of the changed frequency component derived from the receivedwave.

The selected portion of the changed frequency energy is amplified,preferably in one ormore stages each includin a space dis- 'amplitude ofthe incoming wave, from which it is derived, a variable voltage dro'will be produced across the resistance. T is variable voltage is appliedto the high frequency detector to control its gain, in a manner inverseto fluctuations in the incoming wave, whereby these fluctuations arecompensated.

In addition to the variable gain control potential, the various "spacedischarge devices or tubes, which operate as amplifiers and rectifiers,are supplied with steady biasing potentials derived from resistanceelements included' in the circuits through which heating current issupplied to the filaments of the discharge devices.

- In order that the steady biasing potentials and the variablepotentials, used to control thegain of the receiver, may always becontrolled to have the same relative values in spite of changes in theoperating characteristic of the receiver or changes in the circuitsthereof, for example, when a space discharge tube is changed, a pair ofsimultaneously adustable resistances are included in the filamentheating circuit of the filament of one of the discharge tubes.

The resistances function to permit the potential difference between thegrid and filament of the tube, with which they are associated, to bevaried without affecting the potential of the point on the filamentheating circuit whence are derived potentials applied tion, will beclear from the following descrip- In the drawing there is shown anantenna 1 connected through a tuned circuit 2'. and

condenser 3 to'ground. r I a The circuit 2 is tuned to the frequency' ofthe wave to be received. The condenser 3 serves as a low impedance pathto ground for the high frequency ener and to prevent the flow of director low requency current to ground.

High frequency energy, produced in the tuned circuit 2 by the waveincident upon the antenna 1, is supplied to the coil or autotransformer4.

The connection from the antenna circuit is made at an intermediate pointof the coil 4, which has one terminal connected to the grid of modulator5 and its other terminal connected through an adjustable condenser 6 toground.

Oscillations from the source 7, which may be of any type, for example, avacuum tube oscillator, are supplied throu'h a coupling between thecoils 8 and 4 to t e grid of the modulator 5.

The method of connecting the antenna to the tube 5 is similar to thatdisclosed in application, Serial No. 48,988, filed August 8, 1925 by E.Bruce, Patent 1,710,254, April 23, 1929, and it is used for the purposetherein set forth. Briefly its purpose is to permit the source of localoscillations to be conjugately coupled with respect to the antenna. Thisserves two purposes, first, it prevents radiation of the locallysupplied oscillations, and second, it permits the tuning of the circuit2 to be adjusted without causing variations in the amount of energysupplied by the local source to the modulator.

The incoming wave and the locally supplied oscillations are combined inthe device 5 to produce a changed frequency wave. This wave is selectedby the filter 9 and amplified by an amplifier 10 comprising any numberof stages. A

One stage of the changed frequency amplifier is shown in detail at 11,as a distortionless amplifier comprising a space discharge device.However, any other type of distortionless amplifier may be used.

The changed frequency wave is applied to the input electrodes of thedetector 12 in whichit is-detected to .produce the speechcurrents whichare supplied through a transformer or repeating coil 13 to a loadcircuit source 16.

tended to any subscriber of the system.

Space current issupplied to the discharge dev1ces 5, 11 .and 12 from thedirect current For tube 5 the space current path extends from thepositive terminal of source 16 through a conductive path within thefilter 9, anode of tube 5 to its cathode, the left hand resistance 17and via ground to the negative terminal of source 16.

The space current circuit for tube 11 extends from the positive terminalof couroe 16, a conductive path in the diagrammatically shown portion ofthe changed fr uency amplifier, anode of 11 to its catho e, resistance20, cathode of 5, resistance 17 and via ground to the negative terminalof batterv 16.

The tube 12 is supplied with space current over the circuit extendingfrom thepositive terminal of source 16, primary winding of transformer13, anode of 12, its cathode and through ground to the negative terminalof source 16.

As shown in the drawing, heating current is supplied to the filaments oftubes 11 and 5, connected in series, from the negative terminal ofsource 18, resistance19', the filament of tube 11, a portion ofresistance 20, filament of tube 5, a portion of resistance 17 andthrough ground to the positive'terminal of source 18. i

The grid of tube 5 is connected to its filament over the followingdirect current path. From the grid of .tube 5 through the lower half ofcoil4, the inductance of the tuned circuit 2, a resistance 21,resistance 22, the filament of tube 11, a portion of resistance 20 tothe right hand or negative terminal of the filament of tube 5.

The grid is therefore negatively biased by the voltage drop caused bythe flow of filament heating current through the series circuitincluding the filament of tube 11 and the ortion of resistance 20associated with the lament of tube 5.

The grid of tube 11 is negatively biased by the voltage drop due to theflow of filament heating current through that portion of the resistance19 between the contact 23 and its terminal connected to the filamentlead.

The output side of'the changed frequenc amplifier 10 isconnecte'd by alead'24 inclu ing a'blocking condenser 25 to a plural stage auxiliary.Qchanged fre uency amplifier 26. A portion of the change uenc energy isI supplied through the-lead in uding the condenser 25 to the auxiliaryunit 26. After being amplified in unit 26, the changed frequency currentis supplied by a transformer 27 to a rectifier R. The rectifier isherein shown, by way of example, as including a three electrode spacedischarge device 28 having its grid and plate electrodes directlyconnected to constitute, in effect, a two electrode device.

Heating current is supplied to the filament of the rectifying tube 28from a source '29 over the following circuit. From the negative terminalof source 29 through a resistance 30, ripple suppressing filter 31, thefilament, resistance 32 and ground to the positive terminal of source29.

The circuit through which the auxiliary chan ed frequency current isapplied to the reetit ier 28 extends from its grid and plate, throughsecondary winding of transformer 27 condenser 33 to ground and thencethrough resistance 32 to its filament and across t e space path within28 to its grid and plate.

The receivin system is so designed that, when the wave incident upon theantenna has an amplitude of definitely chosen value, speech current ofthe minimum amplitude necessary for satisfactory commercial operation issupplied to the apparatus or line connected to the terminals 15 and thegrids of the various tubes have impressed thereon the steady potentialsreferred to above.

When the system is in 0 erative condition, switch 35-is thrown to theeft to engage contact 36.

Provided the amplitude of the incident wave is greater than is necessaryto produce output speech currents of an amplitude equivalent to theminimum commercially satisfac- 40 tory value, changed frequency currentis supplied to the rectifier R. I

The rectified current produced by rectifier R flows over the followingcircuit. From the grid and plate electrodes of tube 28,

v through the secondary winding of transformer 27, ripple suppressionfilter 34, switch 35, contact 36, resistance 22, filament of tube 11,resistance 20, filament of tube 5, resistance 17 and through ground'toresistance 32,

v filament of tube 28 and across its space path to its grid and anode.

The drop of voltage across the filaments of tubes 11 and 5 and theincluded portion of resistance 20 and resistance 22, due to 56 the flowof the unidirectional component of the rectified current therethrough,will be impressed upon the grid of modulator 5, to supplement thebiasing poiential applied hereto due to flow of heating current.

60 By virtue of the supplementary biasing potential, the overall gain ofthe system is controlled to transmit a desired amplitude of speechcurrent to the apparatus or line connected to terminals 15.

Q Should the amplitude of the incident wave tion, Patent 1,778,750, thes increase or decrease, due to fading efiects, the amplitude of therectified current and the voltage drops across the filaments of tubes 11and 5 and the resistances 20 and 22 will vary correspondingly. Thevoltage applied to the grid of modulator 5 will, therefore, increase ordecrease to decrease or increase the overall gain of the system andthereby maintain constant the amplitude of the speech current suppliedto the apparatus or line connected to terminals 15.

As in the above mentioned Bruce applicaof operation of the control isdetermined bythe resistance (R) and the capacity (C) in shunt to R. Inthe present system, B is the resistance 22 and is of 100,000 ohms. Thecapacity (C) in .shunt to 22, is the combined capacities of condensers3, 40, 39 and 33 which e uals .4 mf. The time constant (RC) is there ore100,000 1o- X 10; second,

which is satisfactory for one operating condition. The value of thecapacity of combined condensers 3, 40, 33 and 39 may be materiallyreduced in practice and hence the speed of operation may be increased.

However, if the incident wave fades to such an extent that the amplitudeof the speech current is below the value necessary for satisfactorycommercial operation, the auxiliary amplifier cuts off to prevent thetransmission of changed frequency current to the rectifier tube 28.Hence no gain control potential is applied to the grid of the modulator5.

The id and plate of tube 28 are connected, y a direct current path, tothe left hand terminal of resistance 19 and, therefore, have applied tothem a negative potential determined by the source 18 and resistance 19.This path includes the resistance 22.

Any direct current flow through the resistance 22 would change thebiasmg potentials applied to the grids of tubes 5 and 11 and therebyinterfere with the proper operation of the gain control mechanism,

For example, if the grid and plate of tube 28 are at a positivepotential with respect to its filament, direct current will flow throughthe resistance 22 and thereby increase the biasing potential applied tothe grid of tube 5. This would decrease the overall gain of the systemand thereby reduce the speech current supplied to the apparatus or lineconnected to terminals 15, to a value which would be unsatisfactory forcommercial operation.

To revent' the flow of direct current 1 throug the resistance 22, underthe conditions stated above, the heating circuit of the filament of tube28 is rovided with resistance units 30 and 32 '0 such value that oneterminal of the filament of tube 28 has applied thereto a negativepotential substantially the same or slightly less than that applied tois grid and plate. Since the tube 28 5 is unilaterally conductive andcurrent can ..only flow across its space path when the grid and plateare positive with respect to its filament, no current will flow throughits output path, including resistance 22, until changed frequencycurrent, adapted to render the grid and plate ositive with respect tothe filament, is supplied to the tube.

However, by maintaining equal potentials upon the respective electrodesof tube 28, i. e. grid, plate and filament, rectified current will flowthrough resistance 22 when the tube is supplied with changed frequencycurrent of the proper polarity to render the id and plate positlve withrespect to the lament.

During the operation of the system it may be necessary or desirable tochange the steady potential applied to the grid of the modulatlng tube5. To effect this function, without distributing the otential relationswith respect to the recti er tube 28, the filament of tube 5 is providedwith two simultaneously adjustable contacts 37 and 38, whichrespecttively engage the resistances 17 and 20.

As thecontacts 37 and 38 are shifted, the amount of resistance 20,connected between its filament and the negative lead of the filament oftube 11 is varied. Consequently the potential difference between thefilament of tube 5 and its grid is changed. At the same time, the amountof resistance 17 included in the filament heating current is changed byan equal amount to maintain the heating current constant.

The method of control, for the biasing pot'ential applied to the grid oftube 5, possesses the advantage that this potential may be varied by asingle adjustment to meet varying conditions of operation, withoutchanging the total current flowing through the heating current circuitof tubes 11 and 5 and without varying the potential applied to the gridand plate of rectifying tube 28.

While one embodiment of the invention has been disclosed and certainspecific details have been described, it is to be understood that thisinvention is not limited thereto but only by the scope of the attachedclaims. i

What is claimed is:

1. In combination a space discharge device including a hot electrode andat least one cold electrode, a circuit through which heating current issupplied to said hot electrode, means for derivin from said heatingcircuit a normally fixe potential and applying it to said coldelectrode, means in the heating circuit for changing the potentialdifference between said hot and cold electrodes while maintainingconstant said normally fixed potential and means actuated by the outputcurrent of said space discharge device for varying said normally fixedpofrom said heating circuit a fixed potential and applying it to thecold electrodes of a plurality. of said discharge devices, and means inthe heating circuit for changing the potential diiference between thehot and cold electrodes of at least two of said plurality of dischargedevices while maintaining constant said fixed otential,-said meansoperatin to change t e potential of said grid and the potential of thehot electrode of another of said space discharge devices.

3. A system comprising space discharge devices each including a hot andat least one cold electrode, a circuit. through which heating current issupplied to said hot electrodes, means for der1ving from said heatingcircuit a fixed potential and applying it to cold electrodes of aplurality of said discharge devices, means for maintaining the hotelectrode of one of said lurality of devices at substantially the xedpotential, and means in the heating circuit for changing the potentialdifference between the hot and cold electrodes of at least one other ofsaid pluralitiyxof devices while maintaining constant the ed potential.

4. A system comprising space discharge devices each includmg a hot andat least one cold electrode, means for connecting certain of saiddevices in cascade, a series circuit through which heating current is suplied to the hot electrodes of said casca e connected devices, means forderiving from said heating circuit a fixed potential and applying it tocold electrodes of one of said certain devices and at least one otherdevice, means for controlling said other device for applying a varyinpotential to the electrode of said certain evice to which the fixedpotential is applied, and means in the heating circuit for changing theotential difference between the hot and co d electrodes of said certaindevice while maintainig constant said fixed potential.

5. A receiver for signal modulated high frequency carnier wavescomprising a space discharge modulating device including a cathode andgrid, a circuit through which heating current is applied to-saidcathode, means for deriving from said heating circuit a fixed potentialand applying it to said id, a source of oscillatlons differing inrequency from the wave to be received and connected to said modulatorfor changing the frequency of said received wave, means for selectivelyamplifyin the wave of changed frequency, means or detecting saidselected wave to produce the signal current means for rectifying aportion of said changed frequency wave, means actuated by said rectifiedcurrent to control the ain of said receiver, and means in said heatlngcircuit for changin the potential difference between said cat ode andsaid grid while maintaining constant said fixed potential.

In witness whereof, we hereunto subsribe our names this 9th day of March1929.

JOHN G. GABRiEL. FRANK A. HUBBARD.

